1. Field of the Invention
The present invention relates to a metal film on a semiconductor integrated circuit (IC) and particularly to a metal film having homogeneous fine grains.
2. Description of the Prior Art
Conventionally, an Al thin film on an IC chip is formed by sputtering. Referring to FIG. 1, a typical sputtering apparatus is shown. In a vacuum vessel 1, an anode plate 3 and a cathode plate 2 are provided opposite to each other with a spacing. The cathode plate 2 serves also as a target for sputtering and it is of the same material as an Al thin film to be formed. A Si wafer 4 is disposed on the anode plate 3 so that an Al thin film 6 is formed on the wafer 1. A DC power supply 5 is connected between the anode plate 3 and the cathode plate 2 so as to supply DC voltage of approximately 300 V to 700 V between these electrode plates. The wafer 4 is heated to approximately 150.degree. C. to 400.degree. C. by a heater 7. The heater 7 is fed by an AC power supply 8. The vacuum vessel 1 comprises a valve 9 for intake of inert Ar gas necessary for sputtering.
In operation, the Si wafer 4 is first positioned on the anode plate 3 in the vacuum vessel 1. Then, the vacuum vessel 1 is evacuated to approximately 10.sup.-7 Torr by a vacuum pump and after that, Ar gas is fed into the vessel 1 through the valve 9. The pressure of the Ar gas in the vessel 1 is controlled to be maintained at approximately 10.sup.-2 to 10.sup.-3 Torr. In order to generate Ar.sup.+ ions by glow discharge, an appropriate voltage is applied between the anode 3 and the cathode 2. During the glow discharge, Ar.sup.+ ions are attracted toward the cathode plate (that is, the Al target) 2 and collide therewith so that Al atoms are sputtered out. These sputtered Al atoms are deposited on the wafer 4 to form an Al polycrystalline film 6.
In the prior art, efforts have been made to decrease grain boundary area by making large the grain size of the Al film 6 because grain boundaries are adversely affected by electromigration. Therefore, not only for densifying the Al film but also for growing grains, the wafer 4 is suitably heated by the heater 7 so as to activate the diffusion of Al atoms.
Referring to FIG. 2, the Al film 6 having a large grain size is illustrated. The Al film 6 is formed on an oxide film 10 on a major surface of the Si wafer 4. Generally, the Al film 6 has an extremely small thickness of approximately 0.5 to 2.0 .mu.m for example and consequently, most of the grains 12 bounded by grain boundaries 13 extend from the bottom surface to the top surface of the film 6.
Recently, it has been made clear that electromigration can be effectively suppressed by adding to Al, a small amount of impurity such as Cu, Ti or Mg. Consequently, it is not necessarily needed to make large the grain size of the Al film 6. On the other side, it is known that the creep strength in view of a long time period is higher with a smaller grain size. Furthermore, an Al film having a small grain size can be patterned with high precision by etching and thus, a film with small-sized grains is suited for fine IC patterning.